Multiple tissue gases can be monitored using a diffusion membrane tipped probe coupled to a mass spectrometer. The tissue with probe in place constitutes a system in which gas tension readings are a function of many factors including local diffusion and convection processes and other characteristics of the organ or tissue being studied, as well as the material and geometry of the diffusion membrane probe. However, the interrelationship of these factors is only partially understood and there is no experimentally verified quantitative model of tissue appropriate to the description of the tissue-probe system. Such a description may permit an expanded diagnostic role of the diffusion membrance probe in evaluating abnormal tissue function. Recently, we have found experimental evidence in support of a compartmental model for the tissue-probe system and have also found that local subcutaneous tissue perfusion relates linearly to a depletion coefficient derived from diffusion membrane probe readings. The study planned here is to further elucidate these results and to develop a model of tissue which interrelates tissue parameters.